In modern integrated technology, communication between a plurality of devices, such as master devices, slaves, etc is often required. A typical example would include communication between a controller in a vehicle and the different sensors, functional and passive systems, illumination systems, etc. For example, communication can be established between two or more nodes or devices via a differential network bus.
An example of a differential communication network, typically used in automotive applications, is a Controller Area Network (CAN), to be used as a way to reduce the amount of copper wiring by signal multiplexing. Over time, requirements have been expanded to the field of electro-magnetic compatibility.
A simple signal transmission scheme is shown in the upper graph 109 of FIG. 1. In common voltage (or recessive status), both the CAN high CANH and CAN low CANL bus lines have a same signal level 101 (e.g. same voltage). At dominant status, a predetermined voltage difference 102 is introduced between the CAN high CANH and CAN low CANL conduction lines. The signals in both lines CANH, CANL can be then compared, and any parasitic signal introduced in both lines is compensated.
The voltage levels on the bus lines should comply with the specification of the differential network in order to guarantee a correct functioning. Moreover, they should be very symmetric, in order to have low electromagnetic emissions via the bus lines. However, in automotive environments, the bus lines usually pick up electromagnetic disturbances, either from e.g. neighboring wires which may carry high currents (or current changes), via the supply lines of the connected devices, via other coupling mechanisms, etc.
The lower graph 110 in FIG. 1 shows a prior art transceiving unit 111 connected to a CAN bus 112. The transceiving unit 111 may be integrated in a device requiring communication (a CAN node). The receiver Rx 113 of the transceiving unit 111 interprets the voltage information on the bus lines CANH, CANL, and converts that to a received signal 114, which may be further used by the device. The device may require transmitting data, instructions or in general signals through the CAN bus 112. The device generates a transmit signal 115, and a transmitter Tx 116 of the transceiving unit 111 converts said signal to voltage levels on the bus lines CANH, CANL of the bus 112. The signal may be interpreted further by receivers of the rest of the connected devices and/or nodes (not pictured) in the system. Rcan 117 represents the bus impedance. They are e.g. discrete resistors in order to give the network a defined condition. They are typically assembled at the first and the last network connection point and they are present between the bus lines CANH, CANL. The total network impedance may be specified with 60 Ohm total.
A node may further include a controller, such as a CPU or a microcontroller, and a network controller, which is usually an integral part of the CPU or microcontroller, for controlling provision of the signals required from instructions and vice versa.
In environments with a high level of electromagnetic noise, the network bus can be affected by the noise. Typically, automobile implementations of communication networks are considered one of these electromagnetically “noisy” environments. For example, undesired high-frequency current spikes in a differential interconnect network may occur, when the wiring harness of the interconnect network is exposed to electromagnetic disturbances.
These disturbances will also be transported to the transceiving unit of connected members of the differential network, and will interact with elements that are part of the transmitter thereof (e.g. diodes). Further, the unit and its elements do not always have an ideal behavior, for example when switching from a conductive mode into a non-conductive mode. This may lead to additional current components and asymmetry of the signal. As a result, the receivers for the detection of the differential signal sent by the transmitters might detect these additional current components and interpret a wrong signal condition. This may happen within a relatively wide frequency range of electromagnetic disturbance. The malfunctioning of the receivers can lead to a wrong system behavior.
U.S. Pat. No. 6,154,061A describes a bus driver having good symmetry. It shows a circuit arrangement including extra transistors which are scaled copies of the transistors used for driving the signal into the bus. These carry the same current, which delivers a high symmetry and low electromagnetic emissions of the bus lines. However, if the main output driving transistors receive incoming EM interferences, the driving signal may be affected, reduced or even cut off.
US 2017/0199837 describes a transmitter unit including a Miller capacitor for edge control, which allows generating a setpoint voltage characteristic on a differential bus, and transmit it via current mirrors to the bus. Although such transmitter unit reduces line-related emissions in the bus, the improvement does not relate to protecting the line against interference, rather only to improvement in asymmetries during switching edges, when switching from dominant to recessive state.
Because of that, there is a need to provide a robust transmitting unit.